This invention relates generally to light emitting diode (LED) circuits, and more particularly to driver circuits for driving LEDs.
In portable radio communication devices it is desirable to prolong the operating time and battery life. To reduce the current drain from the battery it is desirable to develop circuits that achieve the lowest power consumption possible. Among those circuits, the display draws a disproportionate amount of current from the battery. The LED is widely used for back lighting in devices such as cellular phones due to its simpler driving circuit compared with the electroluminescent (EL) and fluorescent lighting its comparably lower cost and noise. However, the power consumption of LEDs is generally higher than the EL lights when multiple LEDs are used. In addition, the use of white LEDs, which is necessary for backlighting color liquid crystal displays (LCDs), incurs power considerations in that white LEDs have higher threshold voltages, which are often higher than the battery voltages. Thus DC-DC converter is required to boost the battery voltage and the overall power efficiency is reduced.
A radio communication device, such as a cellular phone, is typically powered from a battery, such as a lithium-ion battery, having a normal operating voltage of about 3.6 volts. Ideally, the device circuits are powered directly from the battery, however, some circuits such as light emitting diodes (LEDs) used in displays will not operate at this low voltage or provide deteriorated performance when the battery runs down, and it becomes necessary to add a DC-DC converter to step-up the voltage. However, the inductor type of DC-DC converter may have a typical efficiency of 85%, while the charge pump type of DC-DC converter usually has efficiencies less than 50% when the battery internal resistance is considered.
Referring to FIG. 1, a prior art LED inductive boost driver circuit is illustrated as described in U.S. Pat. No. 4,673,865, including an inductive switching power supply 102 to perform a DC-DC conversion. An inductor 104 is connected between a node 106 and a battery 108. A transistor 110 is connected to node 106. The anode of a diode 112 is also connected to node 106 and the cathode is connected to a node 114. A filter capacitor 116 is connected between node 114 and ground. A duty cycle modulator 118 is connected between node 114 and the base of transistor 110.
In operation, duty cycle modulator 118 periodically switches on and off transistor 110. When transistor 110 is switched on, current from battery 108 begins to flow through inductor 104, building up the magnetic field in the inductor as the current increases. When transistor 110 is switched off, the magnetic field collapses and a positive voltage pulse appears at node 106. Because inductor 104 is in series with battery 108, the voltage of the pulse at node 106 is greater than the battery voltage.
Thus, the periodic switching of transistor 110 causes a string of pulses to appear at node 106. These voltage pulses are then rectified and filtered by diode 112 and filter capacitor 116 to produce a multiplied DC voltage at output node 114. To regulate the output voltage, duty cycle modulator 118 samples the output voltage at DC output node 114 and adjusts the duty cycle of transistor 110 so that the DC output voltage remains substantially constant. A current limiting resistor 124 is coupled in series with the LED 122 along with a transistor 126 to control the activation of LED 122 via a control circuit (not shown). Although an improvement in the art, there is voltage drop across diode 112, and power consumed in current limiting resistor 124, which consumes battery power.
Illustrated in FIG. 2 is another prior art LED driver circuit that consumes less battery energy than the device of FIG. 1. The driver circuit uses switching power supply 102, LED 122 and transistor 126 that were previously described in conjunction with FIG. 1. Also, LED 122 and transistor 126 are mutually interconnected as in FIG. 1 and transistor 126 functions to control the activation of LED 122 as previously described. However, a capacitor 202 is connected between the anode of LED 122 and the pulse output node 106. A shunt diode 204 is connected to the junction of capacitor 202 and LED 122.
In operation, during a positive voltage pulse at output node 106, current flows through LED 122 via coupling capacitor 202. The capacitor plate 202a of capacitor 202 begins to charge negatively. Between voltage pulses, i.e. when transistor 110 conducts and momentarily grounds node 106, capacitor plate 202a goes below ground potential. When the negative potential on capacitor plate 202a is sufficient to overcome the small (typically 0.6 Volts) forward voltage drop across diode 204, the diode conducts, substantially discharging capacitor 202. Thus, diode 204 provides a means for discharging capacitor 202 during a portion of each period of the voltage waveform at output node 106.
Unfortunately, the discharge current is lost, lowering efficiency of the driver circuit. Moreover, this device, as well as that of FIG. 2, utilizes an inductor type boost converter to provide a high supply voltage, which increases cost and size of the circuit.
What is needed is a high efficiency LED driver circuit that can drive LEDs requiring higher voltage than available battery power. It would also be of benefit to eliminate the inductive type of boost circuits and the losses associated with current limiting resistors and switching circuits. It would also be advantageous to accomplish this in a low cost, simple circuit architecture.